llvm/lib/DebugInfo/DWARFContext.cpp

//===-- DWARFContext.cpp --------------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "DWARFContext.h"
#include "DWARFDebugArangeSet.h"

#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Dwarf.h"
#include "llvm/Support/Format.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
using namespace dwarf;
using namespace object;

#define DEBUG_TYPE "dwarf"

typedef DWARFDebugLine::LineTable DWARFLineTable;

static void dumpPubSection(raw_ostream &OS, StringRef Name, StringRef Data,
                           bool LittleEndian, bool GnuStyle) {
  OS << "\n." << Name << " contents:\n";
  DataExtractor pubNames(Data, LittleEndian, 0);
  uint32_t offset = 0;
  while (pubNames.isValidOffset(offset)) {
    OS << "length = " << format("0x%08x", pubNames.getU32(&offset));
    OS << " version = " << format("0x%04x", pubNames.getU16(&offset));
    OS << " unit_offset = " << format("0x%08x", pubNames.getU32(&offset));
    OS << " unit_size = " << format("0x%08x", pubNames.getU32(&offset)) << '\n';
    if (GnuStyle)
      OS << "Offset     Linkage  Kind     Name\n";
    else
      OS << "Offset     Name\n";

    while (offset < Data.size()) {
      uint32_t dieRef = pubNames.getU32(&offset);
      if (dieRef == 0)
        break;
      OS << format("0x%8.8x ", dieRef);
      if (GnuStyle) {
        PubIndexEntryDescriptor desc(pubNames.getU8(&offset));
        OS << format("%-8s", dwarf::GDBIndexEntryLinkageString(desc.Linkage))
           << ' ' << format("%-8s", dwarf::GDBIndexEntryKindString(desc.Kind))
           << ' ';
      }
      OS << '\"' << pubNames.getCStr(&offset) << "\"\n";
    }
  }
}

void DWARFContext::dump(raw_ostream &OS, DIDumpType DumpType) {
  if (DumpType == DIDT_All || DumpType == DIDT_Abbrev) {
    OS << ".debug_abbrev contents:\n";
    getDebugAbbrev()->dump(OS);
  }

  if (DumpType == DIDT_All || DumpType == DIDT_AbbrevDwo)
    if (const DWARFDebugAbbrev *D = getDebugAbbrevDWO()) {
      OS << "\n.debug_abbrev.dwo contents:\n";
      D->dump(OS);
    }

  if (DumpType == DIDT_All || DumpType == DIDT_Info) {
    OS << "\n.debug_info contents:\n";
    for (const auto &CU : compile_units())
      CU->dump(OS);
  }

  if ((DumpType == DIDT_All || DumpType == DIDT_InfoDwo) &&
      getNumDWOCompileUnits()) {
    OS << "\n.debug_info.dwo contents:\n";
    for (const auto &DWOCU : dwo_compile_units())
      DWOCU->dump(OS);
  }

  if ((DumpType == DIDT_All || DumpType == DIDT_Types) && getNumTypeUnits()) {
    OS << "\n.debug_types contents:\n";
    for (const auto &TU : type_units())
      TU->dump(OS);
  }

  if ((DumpType == DIDT_All || DumpType == DIDT_TypesDwo) &&
      getNumDWOTypeUnits()) {
    OS << "\n.debug_types.dwo contents:\n";
    for (const auto &DWOTU : dwo_type_units())
      DWOTU->dump(OS);
  }

  if (DumpType == DIDT_All || DumpType == DIDT_Loc) {
    OS << "\n.debug_loc contents:\n";
    getDebugLoc()->dump(OS);
  }

  if (DumpType == DIDT_All || DumpType == DIDT_LocDwo) {
    OS << "\n.debug_loc.dwo contents:\n";
    getDebugLocDWO()->dump(OS);
  }

  if (DumpType == DIDT_All || DumpType == DIDT_Frames) {
    OS << "\n.debug_frame contents:\n";
    getDebugFrame()->dump(OS);
  }

  uint32_t offset = 0;
  if (DumpType == DIDT_All || DumpType == DIDT_Aranges) {
    OS << "\n.debug_aranges contents:\n";
    DataExtractor arangesData(getARangeSection(), isLittleEndian(), 0);
    DWARFDebugArangeSet set;
    while (set.extract(arangesData, &offset))
      set.dump(OS);
  }

  uint8_t savedAddressByteSize = 0;
  if (DumpType == DIDT_All || DumpType == DIDT_Line) {
    OS << "\n.debug_line contents:\n";
    for (const auto &CU : compile_units()) {
      savedAddressByteSize = CU->getAddressByteSize();
      unsigned stmtOffset =
          CU->getCompileUnitDIE()->getAttributeValueAsSectionOffset(
              CU.get(), DW_AT_stmt_list, -1U);
      if (stmtOffset != -1U) {
        DataExtractor lineData(getLineSection().Data, isLittleEndian(),
                               savedAddressByteSize);
        DWARFDebugLine::LineTable LineTable;
        LineTable.parse(lineData, &getLineSection().Relocs, &stmtOffset);
        LineTable.dump(OS);
      }
    }
  }

  if (DumpType == DIDT_All || DumpType == DIDT_LineDwo) {
    OS << "\n.debug_line.dwo contents:\n";
    unsigned stmtOffset = 0;
    DataExtractor lineData(getLineDWOSection().Data, isLittleEndian(),
                           savedAddressByteSize);
    DWARFDebugLine::LineTable LineTable;
    while (LineTable.Prologue.parse(lineData, &stmtOffset)) {
      LineTable.dump(OS);
      LineTable.clear();
    }
  }

  if (DumpType == DIDT_All || DumpType == DIDT_Str) {
    OS << "\n.debug_str contents:\n";
    DataExtractor strData(getStringSection(), isLittleEndian(), 0);
    offset = 0;
    uint32_t strOffset = 0;
    while (const char *s = strData.getCStr(&offset)) {
      OS << format("0x%8.8x: \"%s\"\n", strOffset, s);
      strOffset = offset;
    }
  }

  if ((DumpType == DIDT_All || DumpType == DIDT_StrDwo) &&
      !getStringDWOSection().empty()) {
    OS << "\n.debug_str.dwo contents:\n";
    DataExtractor strDWOData(getStringDWOSection(), isLittleEndian(), 0);
    offset = 0;
    uint32_t strDWOOffset = 0;
    while (const char *s = strDWOData.getCStr(&offset)) {
      OS << format("0x%8.8x: \"%s\"\n", strDWOOffset, s);
      strDWOOffset = offset;
    }
  }

  if (DumpType == DIDT_All || DumpType == DIDT_Ranges) {
    OS << "\n.debug_ranges contents:\n";
    // In fact, different compile units may have different address byte
    // sizes, but for simplicity we just use the address byte size of the last
    // compile unit (there is no easy and fast way to associate address range
    // list and the compile unit it describes).
    DataExtractor rangesData(getRangeSection(), isLittleEndian(),
                             savedAddressByteSize);
    offset = 0;
    DWARFDebugRangeList rangeList;
    while (rangeList.extract(rangesData, &offset))
      rangeList.dump(OS);
  }

  if (DumpType == DIDT_All || DumpType == DIDT_Pubnames)
    dumpPubSection(OS, "debug_pubnames", getPubNamesSection(),
                   isLittleEndian(), false);

  if (DumpType == DIDT_All || DumpType == DIDT_Pubtypes)
    dumpPubSection(OS, "debug_pubtypes", getPubTypesSection(),
                   isLittleEndian(), false);

  if (DumpType == DIDT_All || DumpType == DIDT_GnuPubnames)
    dumpPubSection(OS, "debug_gnu_pubnames", getGnuPubNamesSection(),
                   isLittleEndian(), true /* GnuStyle */);

  if (DumpType == DIDT_All || DumpType == DIDT_GnuPubtypes)
    dumpPubSection(OS, "debug_gnu_pubtypes", getGnuPubTypesSection(),
                   isLittleEndian(), true /* GnuStyle */);

  if ((DumpType == DIDT_All || DumpType == DIDT_StrOffsetsDwo) &&
      !getStringOffsetDWOSection().empty()) {
    OS << "\n.debug_str_offsets.dwo contents:\n";
    DataExtractor strOffsetExt(getStringOffsetDWOSection(), isLittleEndian(),
                               0);
    offset = 0;
    uint64_t size = getStringOffsetDWOSection().size();
    while (offset < size) {
      OS << format("0x%8.8x: ", offset);
      OS << format("%8.8x\n", strOffsetExt.getU32(&offset));
    }
  }
}

const DWARFDebugAbbrev *DWARFContext::getDebugAbbrev() {
  if (Abbrev)
    return Abbrev.get();

  DataExtractor abbrData(getAbbrevSection(), isLittleEndian(), 0);

  Abbrev.reset(new DWARFDebugAbbrev());
  Abbrev->extract(abbrData);
  return Abbrev.get();
}

const DWARFDebugAbbrev *DWARFContext::getDebugAbbrevDWO() {
  if (AbbrevDWO)
    return AbbrevDWO.get();

  DataExtractor abbrData(getAbbrevDWOSection(), isLittleEndian(), 0);
  AbbrevDWO.reset(new DWARFDebugAbbrev());
  AbbrevDWO->extract(abbrData);
  return AbbrevDWO.get();
}

const DWARFDebugLoc *DWARFContext::getDebugLoc() {
  if (Loc)
    return Loc.get();

  DataExtractor LocData(getLocSection().Data, isLittleEndian(), 0);
  Loc.reset(new DWARFDebugLoc(getLocSection().Relocs));
  // assume all compile units have the same address byte size
  if (getNumCompileUnits())
    Loc->parse(LocData, getCompileUnitAtIndex(0)->getAddressByteSize());
  return Loc.get();
}

const DWARFDebugLocDWO *DWARFContext::getDebugLocDWO() {
  if (LocDWO)
    return LocDWO.get();

  DataExtractor LocData(getLocDWOSection().Data, isLittleEndian(), 0);
  LocDWO.reset(new DWARFDebugLocDWO());
  LocDWO->parse(LocData);
  return LocDWO.get();
}

const DWARFDebugAranges *DWARFContext::getDebugAranges() {
  if (Aranges)
    return Aranges.get();

  Aranges.reset(new DWARFDebugAranges());
  Aranges->generate(this);
  return Aranges.get();
}

const DWARFDebugFrame *DWARFContext::getDebugFrame() {
  if (DebugFrame)
    return DebugFrame.get();

  // There's a "bug" in the DWARFv3 standard with respect to the target address
  // size within debug frame sections. While DWARF is supposed to be independent
  // of its container, FDEs have fields with size being "target address size",
  // which isn't specified in DWARF in general. It's only specified for CUs, but
  // .eh_frame can appear without a .debug_info section. Follow the example of
  // other tools (libdwarf) and extract this from the container (ObjectFile
  // provides this information). This problem is fixed in DWARFv4
  // See this dwarf-discuss discussion for more details:
  // http://lists.dwarfstd.org/htdig.cgi/dwarf-discuss-dwarfstd.org/2011-December/001173.html
  DataExtractor debugFrameData(getDebugFrameSection(), isLittleEndian(),
                               getAddressSize());
  DebugFrame.reset(new DWARFDebugFrame());
  DebugFrame->parse(debugFrameData);
  return DebugFrame.get();
}

const DWARFLineTable *
DWARFContext::getLineTableForCompileUnit(DWARFCompileUnit *cu) {
  if (!Line)
    Line.reset(new DWARFDebugLine(&getLineSection().Relocs));

  unsigned stmtOffset =
      cu->getCompileUnitDIE()->getAttributeValueAsSectionOffset(
          cu, DW_AT_stmt_list, -1U);
  if (stmtOffset == -1U)
    return nullptr; // No line table for this compile unit.

  // See if the line table is cached.
  if (const DWARFLineTable *lt = Line->getLineTable(stmtOffset))
    return lt;

  // We have to parse it first.
  DataExtractor lineData(getLineSection().Data, isLittleEndian(),
                         cu->getAddressByteSize());
  return Line->getOrParseLineTable(lineData, stmtOffset);
}

void DWARFContext::parseCompileUnits() {
  if (!CUs.empty())
    return;
  uint32_t offset = 0;
  const DataExtractor &DIData = DataExtractor(getInfoSection().Data,
                                              isLittleEndian(), 0);
  while (DIData.isValidOffset(offset)) {
    std::unique_ptr<DWARFCompileUnit> CU(new DWARFCompileUnit(
        getDebugAbbrev(), getInfoSection().Data, getRangeSection(),
        getStringSection(), StringRef(), getAddrSection(),
        &getInfoSection().Relocs, isLittleEndian()));
    if (!CU->extract(DIData, &offset)) {
      break;
    }
    CUs.push_back(std::move(CU));
    offset = CUs.back()->getNextUnitOffset();
  }
}

void DWARFContext::parseTypeUnits() {
  if (!TUs.empty())
    return;
  for (const auto &I : getTypesSections()) {
    uint32_t offset = 0;
    const DataExtractor &DIData =
        DataExtractor(I.second.Data, isLittleEndian(), 0);
    while (DIData.isValidOffset(offset)) {
      std::unique_ptr<DWARFTypeUnit> TU(
          new DWARFTypeUnit(getDebugAbbrev(), I.second.Data, getRangeSection(),
                            getStringSection(), StringRef(), getAddrSection(),
                            &I.second.Relocs, isLittleEndian()));
      if (!TU->extract(DIData, &offset))
        break;
      TUs.push_back(std::move(TU));
      offset = TUs.back()->getNextUnitOffset();
    }
  }
}

void DWARFContext::parseDWOCompileUnits() {
  if (!DWOCUs.empty())
    return;
  uint32_t offset = 0;
  const DataExtractor &DIData =
      DataExtractor(getInfoDWOSection().Data, isLittleEndian(), 0);
  while (DIData.isValidOffset(offset)) {
    std::unique_ptr<DWARFCompileUnit> DWOCU(new DWARFCompileUnit(
        getDebugAbbrevDWO(), getInfoDWOSection().Data, getRangeDWOSection(),
        getStringDWOSection(), getStringOffsetDWOSection(), getAddrSection(),
        &getInfoDWOSection().Relocs, isLittleEndian()));
    if (!DWOCU->extract(DIData, &offset)) {
      break;
    }
    DWOCUs.push_back(std::move(DWOCU));
    offset = DWOCUs.back()->getNextUnitOffset();
  }
}

void DWARFContext::parseDWOTypeUnits() {
  if (!DWOTUs.empty())
    return;
  for (const auto &I : getTypesDWOSections()) {
    uint32_t offset = 0;
    const DataExtractor &DIData =
        DataExtractor(I.second.Data, isLittleEndian(), 0);
    while (DIData.isValidOffset(offset)) {
      std::unique_ptr<DWARFTypeUnit> TU(new DWARFTypeUnit(
          getDebugAbbrevDWO(), I.second.Data, getRangeDWOSection(),
          getStringDWOSection(), getStringOffsetDWOSection(), getAddrSection(),
          &I.second.Relocs, isLittleEndian()));
      if (!TU->extract(DIData, &offset))
        break;
      DWOTUs.push_back(std::move(TU));
      offset = DWOTUs.back()->getNextUnitOffset();
    }
  }
}

namespace {
  struct OffsetComparator {

    bool operator()(const std::unique_ptr<DWARFCompileUnit> &LHS,
                    const std::unique_ptr<DWARFCompileUnit> &RHS) const {
      return LHS->getOffset() < RHS->getOffset();
    }
    bool operator()(const std::unique_ptr<DWARFCompileUnit> &LHS,
                    uint32_t RHS) const {
      return LHS->getOffset() < RHS;
    }
    bool operator()(uint32_t LHS,
                    const std::unique_ptr<DWARFCompileUnit> &RHS) const {
      return LHS < RHS->getOffset();
    }
  };
}

DWARFCompileUnit *DWARFContext::getCompileUnitForOffset(uint32_t Offset) {
  parseCompileUnits();

  std::unique_ptr<DWARFCompileUnit> *CU =
      std::lower_bound(CUs.begin(), CUs.end(), Offset, OffsetComparator());
  if (CU != CUs.end()) {
    return CU->get();
  }
  return nullptr;
}

DWARFCompileUnit *DWARFContext::getCompileUnitForAddress(uint64_t Address) {
  // First, get the offset of the compile unit.
  uint32_t CUOffset = getDebugAranges()->findAddress(Address);
  // Retrieve the compile unit.
  return getCompileUnitForOffset(CUOffset);
}

static bool getFileNameForCompileUnit(DWARFCompileUnit *CU,
                                      const DWARFLineTable *LineTable,
                                      uint64_t FileIndex,
                                      bool NeedsAbsoluteFilePath,
                                      std::string &FileName) {
  if (!CU || !LineTable ||
      !LineTable->getFileNameByIndex(FileIndex, NeedsAbsoluteFilePath,
                                     FileName))
    return false;
  if (NeedsAbsoluteFilePath && sys::path::is_relative(FileName)) {
    // We may still need to append compilation directory of compile unit.
    SmallString<16> AbsolutePath;
    if (const char *CompilationDir = CU->getCompilationDir()) {
      sys::path::append(AbsolutePath, CompilationDir);
    }
    sys::path::append(AbsolutePath, FileName);
    FileName = AbsolutePath.str();
  }
  return true;
}

static bool getFileLineInfoForCompileUnit(DWARFCompileUnit *CU,
                                          const DWARFLineTable *LineTable,
                                          uint64_t Address,
                                          bool NeedsAbsoluteFilePath,
                                          DILineInfo &Result) {
  if (!CU || !LineTable)
    return false;
  // Get the index of row we're looking for in the line table.
  uint32_t RowIndex = LineTable->lookupAddress(Address);
  if (RowIndex == -1U)
    return false;
  // Take file number and line/column from the row.
  const DWARFDebugLine::Row &Row = LineTable->Rows[RowIndex];
  if (!getFileNameForCompileUnit(CU, LineTable, Row.File, NeedsAbsoluteFilePath,
                                 Result.FileName))
    return false;
  Result.Line = Row.Line;
  Result.Column = Row.Column;
  return true;
}

static bool getFunctionNameForAddress(DWARFCompileUnit *CU, uint64_t Address,
                                      std::string &FunctionName) {
  // The address may correspond to instruction in some inlined function,
  // so we have to build the chain of inlined functions and take the
  // name of the topmost function in it.
  const DWARFDebugInfoEntryInlinedChain &InlinedChain =
      CU->getInlinedChainForAddress(Address);
  if (InlinedChain.DIEs.size() == 0)
    return false;
  const DWARFDebugInfoEntryMinimal &TopFunctionDIE = InlinedChain.DIEs[0];
  if (const char *Name = TopFunctionDIE.getSubroutineName(InlinedChain.U)) {
    FunctionName = Name;
    return true;
  }
  return false;
}

DILineInfo DWARFContext::getLineInfoForAddress(uint64_t Address,
    DILineInfoSpecifier Specifier) {
  DILineInfo Result;

  DWARFCompileUnit *CU = getCompileUnitForAddress(Address);
  if (!CU)
    return Result;
  if (Specifier.needs(DILineInfoSpecifier::FunctionName)) {
    getFunctionNameForAddress(CU, Address, Result.FunctionName);
  }
  if (Specifier.needs(DILineInfoSpecifier::FileLineInfo)) {
    const DWARFLineTable *LineTable = getLineTableForCompileUnit(CU);
    const bool NeedsAbsoluteFilePath =
        Specifier.needs(DILineInfoSpecifier::AbsoluteFilePath);
    getFileLineInfoForCompileUnit(CU, LineTable, Address, NeedsAbsoluteFilePath,
                                  Result);
  }
  return Result;
}

DILineInfoTable DWARFContext::getLineInfoForAddressRange(uint64_t Address,
    uint64_t Size,
    DILineInfoSpecifier Specifier) {
  DILineInfoTable  Lines;
  DWARFCompileUnit *CU = getCompileUnitForAddress(Address);
  if (!CU)
    return Lines;

  std::string FunctionName = "<invalid>";
  if (Specifier.needs(DILineInfoSpecifier::FunctionName)) {
    getFunctionNameForAddress(CU, Address, FunctionName);
  }

  // If the Specifier says we don't need FileLineInfo, just
  // return the top-most function at the starting address.
  if (!Specifier.needs(DILineInfoSpecifier::FileLineInfo)) {
    DILineInfo Result;
    Result.FunctionName = FunctionName;
    Lines.push_back(std::make_pair(Address, Result));
    return Lines;
  }

  const DWARFLineTable *LineTable = getLineTableForCompileUnit(CU);
  const bool NeedsAbsoluteFilePath =
      Specifier.needs(DILineInfoSpecifier::AbsoluteFilePath);

  // Get the index of row we're looking for in the line table.
  std::vector<uint32_t> RowVector;
  if (!LineTable->lookupAddressRange(Address, Size, RowVector))
    return Lines;

  for (uint32_t RowIndex : RowVector) {
    // Take file number and line/column from the row.
    const DWARFDebugLine::Row &Row = LineTable->Rows[RowIndex];
    DILineInfo Result;
    getFileNameForCompileUnit(CU, LineTable, Row.File, NeedsAbsoluteFilePath,
                              Result.FileName);
    Result.FunctionName = FunctionName;
    Result.Line = Row.Line;
    Result.Column = Row.Column;
    Lines.push_back(std::make_pair(Row.Address, Result));
  }

  return Lines;
}

DIInliningInfo DWARFContext::getInliningInfoForAddress(uint64_t Address,
    DILineInfoSpecifier Specifier) {
  DIInliningInfo InliningInfo;

  DWARFCompileUnit *CU = getCompileUnitForAddress(Address);
  if (!CU)
    return InliningInfo;

  const DWARFLineTable *LineTable = nullptr;
  const bool NeedsAbsoluteFilePath =
      Specifier.needs(DILineInfoSpecifier::AbsoluteFilePath);
  const DWARFDebugInfoEntryInlinedChain &InlinedChain =
      CU->getInlinedChainForAddress(Address);
  if (InlinedChain.DIEs.size() == 0) {
    // If there is no DIE for address (e.g. it is in unavailable .dwo file),
    // try to at least get file/line info from symbol table.
    if (Specifier.needs(DILineInfoSpecifier::FileLineInfo)) {
      DILineInfo Frame;
      LineTable = getLineTableForCompileUnit(CU);
      if (getFileLineInfoForCompileUnit(CU, LineTable, Address,
                                        NeedsAbsoluteFilePath, Frame)) {
        InliningInfo.addFrame(Frame);
      }
    }
    return InliningInfo;
  }

  uint32_t CallFile = 0, CallLine = 0, CallColumn = 0;
  for (uint32_t i = 0, n = InlinedChain.DIEs.size(); i != n; i++) {
    const DWARFDebugInfoEntryMinimal &FunctionDIE = InlinedChain.DIEs[i];
    DILineInfo Frame;
    // Get function name if necessary.
    if (Specifier.needs(DILineInfoSpecifier::FunctionName)) {
      if (const char *Name = FunctionDIE.getSubroutineName(InlinedChain.U))
        Frame.FunctionName = Name;
    }
    if (Specifier.needs(DILineInfoSpecifier::FileLineInfo)) {
      if (i == 0) {
        // For the topmost frame, initialize the line table of this
        // compile unit and fetch file/line info from it.
        LineTable = getLineTableForCompileUnit(CU);
        // For the topmost routine, get file/line info from line table.
        getFileLineInfoForCompileUnit(CU, LineTable, Address,
                                      NeedsAbsoluteFilePath, Frame);
      } else {
        // Otherwise, use call file, call line and call column from
        // previous DIE in inlined chain.
        getFileNameForCompileUnit(CU, LineTable, CallFile,
                                  NeedsAbsoluteFilePath, Frame.FileName);
        Frame.Line = CallLine;
        Frame.Column = CallColumn;
      }
      // Get call file/line/column of a current DIE.
      if (i + 1 < n) {
        FunctionDIE.getCallerFrame(InlinedChain.U, CallFile, CallLine,
                                   CallColumn);
      }
    }
    InliningInfo.addFrame(Frame);
  }
  return InliningInfo;
}

static bool consumeCompressedDebugSectionHeader(StringRef &data,
                                                uint64_t &OriginalSize) {
  // Consume "ZLIB" prefix.
  if (!data.startswith("ZLIB"))
    return false;
  data = data.substr(4);
  // Consume uncompressed section size (big-endian 8 bytes).
  DataExtractor extractor(data, false, 8);
  uint32_t Offset = 0;
  OriginalSize = extractor.getU64(&Offset);
  if (Offset == 0)
    return false;
  data = data.substr(Offset);
  return true;
}

DWARFContextInMemory::DWARFContextInMemory(object::ObjectFile *Obj)
    : IsLittleEndian(Obj->isLittleEndian()),
      AddressSize(Obj->getBytesInAddress()) {
  for (const SectionRef &Section : Obj->sections()) {
    StringRef name;
    Section.getName(name);
    StringRef data;
    Section.getContents(data);

    name = name.substr(name.find_first_not_of("._")); // Skip . and _ prefixes.

    // Check if debug info section is compressed with zlib.
    if (name.startswith("zdebug_")) {
      uint64_t OriginalSize;
      if (!zlib::isAvailable() ||
          !consumeCompressedDebugSectionHeader(data, OriginalSize))
        continue;
      UncompressedSections.resize(UncompressedSections.size() + 1);
      if (zlib::uncompress(data, UncompressedSections.back(), OriginalSize) !=
          zlib::StatusOK) {
        UncompressedSections.pop_back();
        continue;
      }
      // Make data point to uncompressed section contents and save its contents.
      name = name.substr(1);
      data = UncompressedSections.back();
    }

    StringRef *SectionData =
        StringSwitch<StringRef *>(name)
            .Case("debug_info", &InfoSection.Data)
            .Case("debug_abbrev", &AbbrevSection)
            .Case("debug_loc", &LocSection.Data)
            .Case("debug_line", &LineSection.Data)
            .Case("debug_aranges", &ARangeSection)
            .Case("debug_frame", &DebugFrameSection)
            .Case("debug_str", &StringSection)
            .Case("debug_ranges", &RangeSection)
            .Case("debug_pubnames", &PubNamesSection)
            .Case("debug_pubtypes", &PubTypesSection)
            .Case("debug_gnu_pubnames", &GnuPubNamesSection)
            .Case("debug_gnu_pubtypes", &GnuPubTypesSection)
            .Case("debug_info.dwo", &InfoDWOSection.Data)
            .Case("debug_abbrev.dwo", &AbbrevDWOSection)
            .Case("debug_loc.dwo", &LocDWOSection.Data)
            .Case("debug_line.dwo", &LineDWOSection.Data)
            .Case("debug_str.dwo", &StringDWOSection)
            .Case("debug_str_offsets.dwo", &StringOffsetDWOSection)
            .Case("debug_addr", &AddrSection)
            // Any more debug info sections go here.
            .Default(nullptr);
    if (SectionData) {
      *SectionData = data;
      if (name == "debug_ranges") {
        // FIXME: Use the other dwo range section when we emit it.
        RangeDWOSection = data;
      }
    } else if (name == "debug_types") {
      // Find debug_types data by section rather than name as there are
      // multiple, comdat grouped, debug_types sections.
      TypesSections[Section].Data = data;
    } else if (name == "debug_types.dwo") {
      TypesDWOSections[Section].Data = data;
    }

    section_iterator RelocatedSection = Section.getRelocatedSection();
    if (RelocatedSection == Obj->section_end())
      continue;

    StringRef RelSecName;
    RelocatedSection->getName(RelSecName);
    RelSecName = RelSecName.substr(
        RelSecName.find_first_not_of("._")); // Skip . and _ prefixes.

    // TODO: Add support for relocations in other sections as needed.
    // Record relocations for the debug_info and debug_line sections.
    RelocAddrMap *Map = StringSwitch<RelocAddrMap*>(RelSecName)
        .Case("debug_info", &InfoSection.Relocs)
        .Case("debug_loc", &LocSection.Relocs)
        .Case("debug_info.dwo", &InfoDWOSection.Relocs)
        .Case("debug_line", &LineSection.Relocs)
        .Default(nullptr);
    if (!Map) {
      // Find debug_types relocs by section rather than name as there are
      // multiple, comdat grouped, debug_types sections.
      if (RelSecName == "debug_types")
        Map = &TypesSections[*RelocatedSection].Relocs;
      else if (RelSecName == "debug_types.dwo")
        Map = &TypesDWOSections[*RelocatedSection].Relocs;
      else
        continue;
    }

    if (Section.relocation_begin() != Section.relocation_end()) {
      uint64_t SectionSize;
      RelocatedSection->getSize(SectionSize);
      for (const RelocationRef &Reloc : Section.relocations()) {
        uint64_t Address;
        Reloc.getOffset(Address);
        uint64_t Type;
        Reloc.getType(Type);
        uint64_t SymAddr = 0;
        // ELF relocations may need the symbol address
        if (Obj->isELF()) {
          object::symbol_iterator Sym = Reloc.getSymbol();
          Sym->getAddress(SymAddr);
        }

        object::RelocVisitor V(Obj->getFileFormatName());
        // The section address is always 0 for debug sections.
        object::RelocToApply R(V.visit(Type, Reloc, 0, SymAddr));
        if (V.error()) {
          SmallString<32> Name;
          error_code ec(Reloc.getTypeName(Name));
          if (ec) {
            errs() << "Aaaaaa! Nameless relocation! Aaaaaa!\n";
          }
          errs() << "error: failed to compute relocation: "
                 << Name << "\n";
          continue;
        }

        if (Address + R.Width > SectionSize) {
          errs() << "error: " << R.Width << "-byte relocation starting "
                 << Address << " bytes into section " << name << " which is "
                 << SectionSize << " bytes long.\n";
          continue;
        }
        if (R.Width > 8) {
          errs() << "error: can't handle a relocation of more than 8 bytes at "
                    "a time.\n";
          continue;
        }
        DEBUG(dbgs() << "Writing " << format("%p", R.Value)
                     << " at " << format("%p", Address)
                     << " with width " << format("%d", R.Width)
                     << "\n");
        Map->insert(std::make_pair(Address, std::make_pair(R.Width, R.Value)));
      }
    }
  }
}

void DWARFContextInMemory::anchor() { }